Chrome-magnesia refractory and method



April 13, 1943. J. BERLEK CHROME-MAGNESIA REFRACTORY AND METHOD Filed Jan. 8, 1958 BSEE 5 53% @833 5 N w :8 mm o R u E EWHRQa.

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Patented Apr. 13, 1943 onnommmoxnsm nnrmoroar AND METHOD Josei' Berlek, Rad'enthein, Carinthia, Austria:

vested in the Alien Application January 8, 1938, Serial In Austria April 28, 1937 9 Claims. My invention relates to the production of reiractories, and more particularly of shaped re-' iractory bricks, from mixtures of chromite and a highly magnesian refractory vehicle, and to methods of making the same. My inventionre- Property Custodian lates both to the processes involved and to the brick produced.

Whenever I use the term chromite in the specification and in the appended claims I mean chrome iron ore (chrome ore), a mineral of the spine] group containing as the main constituent, besides some gangue, a double oxide of iron is FeQOnOa, pant of the-chromium oxide being always replaced by ferric oxide and alumina, and the iron oxide being always partially replaced by magnesium oxide. Though in the scientific sense the term chromite means the pure compound FeO.CrzO2, in technical language the terms chromite and chrome iron ore are used as synonyms to denote the native mineral oi the spinel group as specified above.

The term magnesia is used herein and in the claims to mean highly magnesian refractory vehicle of any kind, and particularly to include not only calcined magnesite but also fused magnesia.

The main object 01' my invention is to provide bricks which combine to an eminent extent the qualities of strengthat high temperatures, resistance to sudden changes of temperature, and

resistance to chemical influences, while having a nesia, and that hand in hand therewith the resistance to sudden changes of temperature increases also; To achieve a satisfactory improvement in resistance to sudden changes of temperature, in this manner, however, the quantity of magnesia added must-be so great that the slag resistance of the mix is thereby greatly depreciated. Moreover, the production 01' chromemagnesia bricks which would meet more or less equally well all the demands made of retrac- 'and chromium, the chemical formula 01' which tories is confronted with the difliculty that the addition of slight quantities oi magnesia to chromite has a very unfavorable influence upon the strength at atmospheric temperatures of the bricks made from the mix. By comparative tests with mixtures of chromite and different propor- ,tions or magnesite, starting at 10 per cent of magnesia and 90 per cent of chromite and ending at 90 per cent of magnesia and 10 per cent of chromite, it has been found that the curve of strength at atmospheric temperatures falls oil abruptly from the very start up to a magnesium content of 10 to 20 per cent and only thereafter has a pronounced upward tendency.

Various courses have been adopted with the view of overcoming these obstacles sister as possible.

Thus it has been proposed to add to the chromite preponderant amounts of magnesia (preferably 70 per cent) and to counteract the considerable falling oil of the strength at atmospheric temperatures which for some reason or other has been found to be attendant on the low melting silicates higher fusing silicates, i-t

chemical analysis and practising of this proposal, by introducing the magnesia in the form or calcareous magnesia clinker (resulting from dolomitic magnesite). In this manner there are obtained fired bricks which are highly resistant to sudden changes of temperature, but which on account of containing magnesite in excess, are unavoidably greatly inferior to chromite, bricks containing a smaller proportion of magnesia, as far as resistance to acid slags is concerned, which drawback is still further increased by the addition of the considerable amount. 0!.lime peculiar to this known process.

Moreover, proceeding from the assumption that the favorable influence of an addition of finely divided magnesia to the due to the fact that this present in the system Mg0.Si0: into more richly magnesiferous and was proposed to add to finely divided magnesia strength of preliminary petrographic examination, to convert the silicates contained in the gangue into the orthosilicate 2MgO.SiO2 (known as f0rsterite") without the formation of watersoluble magnesium chromate. For this purpose it should generally speaking be suflicient to use quantities in excess oi 12.5

the chrome ore as much as just suflices, on the to between 17 and 25 per cent. At the same time, according to this proposal, the chrome ore should be subjected chromite starting mix is basic oxide converts the per cent, and preferduced into the sisting in burning the ore, preferably in cpmmixture with a portion or the whole of the quantity of magnesia to be added, at a high temperature between 1550 and 1920 C. (preferably between 1760 and 1870 C.). In this manner it has been stated to have been possible to obtain bricks of highly increased resistance to spalling from a mixture of chromite with magnesite containing preponderant amounts of chromite and, therefore, showing very fair slag resistance; but this result was conditional on the use of a complicated course of manufacture including repeated burning at very high temperatures, and double disintegrating and grading of the materials to be mixed. 4

Some time ago the object of producing reall the known processes for manufacturin chrome-magnesia bricks be passed in review it is found that knowledge of the cruciaifact has.

nature of this influence may be.

' quantity fractory chrome-magnesia bricks of great resistance to sudden changes of temperature together with high slag resistance was achieved by combining the addition of a minor proportion of finely divided magnesia with the use of certain grain size selection. According to this method the rule for the proportioning of the grain sizes is that the charge be composed of a fine fraction of a grain size below 0.1 mm. (passing through a 150 mesh per linear inch screen), and of a coarse fraction of a grain size above 1.0

mm. (retained on a 16 mesh per linear inch screen), either with the addition of an intermediate fraction in such a manner that the ratio of fine fraction to intermediate fraction to coarse fraction is within the limits of (20 to 40) :(15 to to 65), or in the absence of any intermediate fraction in such a manner that the ratio of the fine fraction to the coarse fraction is within the limits of (20 to :(80 to 60).

I have now found that it is possible to achieve the same valuable result in a still simpler manner, The essence of my present process consists in practically avoiding the presence of fine chromite meal in the batch when producing chrome-magnesia bricks from a mixture of chromite with at the most its own quantity of magnesia, the bulk of this magnesia being intromix in the form of extremely fine meal, In the light of this new knowledge it is sufficient to eliminate from the ground chrome ore containing all grain sizes from 0 to 3 mm. or higher, the finest fraction up to a limit of at least 0.1 mm. (passing through a 150 mesh per linear inch screen) and of 0.5 mm. (passing through a 20 mesh per linear inch screen) at the utmost. Provided this condition be observed no further grading of the chrome ore as to size need be effected. In this manner the result is achieved of surprisingly improving the spalling properties of chrom'te bricks without impairing their slag resistance, and at the same time of increasing the strength at high temperatures while maintaining fair strength at atmospheric temperatures, in a manufacturing process requiring only one burning operation, by simply taking away a relatively small fraction from the chromite, that is to.say with but a very small loss of prime material. It will at once be clear that having achieved this result means a great advance in the art.

A theoretical explanation is not ofiered since only hypotheses are as yet available. This much is beyond question that, given the absence of finely divided chromite, the chemical reaction to be brought about by the added finely divided magnesia is achieved with a single firing. If 7 Within the limits of 0.1 and 0.5 mm. which define the fraction of chromite meal to be eliminatedaccording tothe present invention, it is to be understood that inter alia particularly the and degree of fineness of the added magnesia, have a determining influence on the upper grain size limit (in no case, of course, exceeding 0.5 mm.) actually to be observed in any particular instance. It is an accepted fact that s in manufacturing magnesia bricks from a mixture of coarse ground material with fine magnesite meal the effects of the addition of magnesia become more and more favorable the higher the degree of fineness of the magnesite up to the point of colloidal dispersion. Thisis true also of the present method, as indeed it is of all reactions required to proceed in the solid (as opposed to the melted) phase. The degree of fineness of the added magnesia, and more particularly the relative proportion of extremely fine particles, is of decisive importance for the present process. The magnesite of grain size from 0 to 0.1 mm. tube embodied in the mix is therefore preferably converted into extremely fine meal by further comminution. Provided this be done the upper limit of the chromite fraction to be taken away may be far below 0.8 mm. As long as the amount and fineness of the magnesia added remain the same, by raising the upper limit of the eliminated chromite fraction the resistance to sudden changes of temperature is improved, especially as regards the uniformity of the results obtained; within the lowest portion of the contemplated range, say within 0.1 to 0.2 or 0.3 mm., however, very small differences in the upward sense bring about pronounced improvement of the spalling qualities, provided extremely finely ground magnesite be employed.

The significance of the upper limit is shown by the accompanying graph illustrative of the conditions obtaining, when the mass contains '70 per cent of chromite and 30 per cent of magnesite, with gradual raising of the upper grain size limit of the chromite fraction removed. It will be seen that with the kind of chromite used in the tests on which this graph is based, the strength at atmospheric temperatures falls off from the very beginning and reaches a range of depression when the upper grain size limit of the fraction to be eliminated is about 1 mm. On the other hand it will be seen that the resistance to sudden changes of temperature reaches its culmination while said upper limit is still at or in the neighborhood of 0.2 mm. and does not increase with further increase of said upper limit. (It must be noted that all the tests in question were discontinued assoon as 70 repetitions of the spalling test cycle had been endured without damage. Thus the curve shows at what point this state is reached.) The favorable conditions thus lie within the range below 0.8 mm., there being, of course, every reason to keep within the lowest portion'of this range for obviously the less chromite to be taken away the better, and this the more so since hand in handwith enlargement of the removed chromite portion the strength at atmospheric pheric temperature, the curve again takes an upward course so that-in the further portions of the range covered by the tests strength at atmospheric temperature improves as the upper limit of the eliminated fraction is raised. There is, however, clearly no wisdom in tolerating a very great'increase in the proportion of prime material to be eliminated, seeing that by doing so the spelling qualities of the refractory are by no means improved, and that in the lowermost portion of the range there are obtained the same and even better results as to strength at atmospheric temperature.

The particular effects due to the absence of finest chromite particles in conJunction with the addition of very finely divided magnesia to the chromite starting mass are achieved even when the quantities of magnesia added amount to only 20 per cent of the total quantity of the mix. If

the proportion of magnesia used falls below 20 per cent the resistance of the bricks to sudden changes of temperature becomes noticeably depreciated and at the same time the strength at ordinary temperature approaches 'the lowest value which is generally reached witha magnesite content of about 10 per cent. The most favorable quantity of magnesia to use, which depends upon various circumstances (e.' g. on the CH: and MgO content of the chromite, on the amount of any surplus sesquioxides present, and

on .the quantity and nature of the silicates contained in the chromite), is preferably determined in any particular instance by experiment.

The magnesite used is obtained by calcining crude magnesite, preferably by burning the same to the point of sintering, in the manner usually adopted in the production of dead-burned magnesite, and that preferably from magnesite poor in lime (0.5 to 2.5 per cent CaO). Instead of deadburned magnesite 'it is also possible'to employ fused magnesia. Of the available chromites hard varieties of chrome-iron ore are suitable in which the CrzOa content is approximately between 38 and 50 per cent. The varieties richer in chrome can only be taken into consideration to a less extent on account of their higher price. The present method renders it possible to work up the chromite in an unburned condition with very favorable results: this does not mean, however, that a pre-burning of the chromite is precluded.

The mix obtained by intimatelycommixing the non-burned ground chromite with the fine magnesia meal is molded, after the addition of water and, if desired, with the use of a binder, dried,

and finally baked, for instance at temperatures between 1400 and 1600 C. As binding agents there may be employed sulphite waste liquor, molasses, dextrine, or any other of the commonly used organic binders, although an inorganic binder such as water glass, magnesium sulphate, or the like, may be used.

Cold-set, non-baked chrome-magnesia bricks may also be produced with advantage by the method according to the present invention.

Examples Conventional grinding practice is used as far as the grinding of the chrome iron ore is concerned. From the ground ore, for instance containing all grain sizes from 0 to 3 mm.-there is removed the fraction comprising the grain sizes (1) 0 to 0.1 mm., (2) 0 to 0.2 mm., (3) 0 to 0.3 mm.,

0 to 0,1 mm., and then finely grinding said fraction with the aid of a tube mill. The grain analysis of such magnesite made with a Gonells air sitterv may give, for example, the following figures:

Millimeters It will be seen that the particles below a size of 0.06 mm. (passing through a 250 mesh per linear inch screen) are largely in excess of the finely ground magnesite meal, and that nearly half the meal consists of particles not exceeding 0.02 mm.

there'are given the figures obtained with a test 7 piece made from the ground chromite containing all grain sizes from 0 to 3 mm., shows the effect of the grain size elimination:

R. 'S. T.=resistance to sudden changes of temperature S. A. T.=strength at atmospheric temperatures S. H. T.=strength at high temperatures ta=temp. at which subsidence commenced te=temp. at which test piece failed.

chromite, T; S. H. '1. Bulk density grain sizes of g S. A. T.

tests m u W i r P;

On removal from the ground chromite material a of the particles up to a grain size of 0.8 mm. the

strength of the bricks at atmospheric temperature falls ofi' still further, down to about 173, under the working conditions specified in the above examples body is kept in the furnace chamber, which is maintained at a uniform temperature of 950 C.,

absorption (W) and bulk density (1') in accordance with the formula Ps=r.W.

- testing for R. S. T. (spalling test) the tested of ground chromite freed temperatures, and temperature, composed of at least 60 per cent half Of which I use the term baking in the following claims to include both kiln firing; and heating to firing temperature in a furnace lining during use without previous kiln firing. j

All percentages mentioned herein are percentages by weight unless the context clearly indicates that they are percentages by volume, as in the case of porosity. Referring to a certain percentage of water soluble bond, I include the water which is used as a vehicle for the bond. 4

The calcined magnesite used may contain between 0.5 and 2.5 per cent CaO. Atypical analysis of a suitable dead-burned magnesite is:

- asiaaao 4. Refractory chromite-magnesia brick having great spailing resistance, slag resistance and strength at high temperatures, and fair strength at atmospheric temperature, composed essentially of about to 40 parts of magnesia meal reduced by special fine grinding to such a degree of fineness that it preponderantly contains particles not exceeding a size of 0.06 mm., and of 60 to A 80 parts of chromite freed from the finest fraction up to an upper limit or at the least 0.1 mm. and at the most 0.5 mm.

5. Refractory chromite-magnesia brick having a great spalling resistance,

slag resistance and strength at high temperatures, and fair strength at atmospheric temperature, composed essentially of about parts of magnesia meal reduced by special fine grinding to such a degree of fineness that nearly half of it is in a state of initial particle size not exceeding 0.02 mm., and IO-parts of chromite freed from the finest fraction up to an upper limit of at the least 0.1 mm. and atthe most 05 mm.

6. In the manufacture of bricks from a mixture of raw chromite and magnesia the process which comprises grinding the chromite, eliminating from the ground material substantially thewhole of the finest fraction which goes at least up to a grain size of 0.1 mm. and does not go beyond 0.5 mm., mixing from 50 to 80 parts of the residual batch with from 50 to 20 parts of finely divided magnesia a preponderant part of which is in a fine state of initial particle size below 0.06 mm., and with a bonding agent,

' and molding the mixture into bricks under pres- Per cent MgO 88.90 69.0 3.00 FeaCa I 3.80 AhOs 1.10 510: 3.10 Loss on ignition 0.10

100.00 A typical analysis of a suitable chromite is:

Per cent CraOa 39.21 S102 4.29 FeaOa 16.75 A1203 22.12 mm 0.19 CaO 0.1 MgO 15.7 Loss on ignition 1.64' 100.00

What I claim is:

1. In the process of making chromite-magnesia bricks having great spalling resistance, slag resistance and strength at high temperatures, and fair strength at atmospheric temperature, the

improvement which comprises composing a charge consisting essentially of ground chromite and ground magnesia and preponderantly containing chromite, in such a manner that the mix will contain about 20 to 40 parts of magnesia meal reduced by special fine grinding to such a. degree of fineness that it preponderantly contains particles not exceeding a size of 0.06 mm., and about 60 to 80 parts of chromite substantially freed from the finest particles up to an upper limit of at the least about 0.1 mm. and at the most about 0.5 mm.; adding a binding substance, molding the mixture into brick and baking the molded bodies.

2. Refractory brick having great spalling resistance, slag resistance and strength at high temperatures, and fair strength at atmospheric temperature, composed of at least 50 per cent from the finest fraction up to an upper limit'of at the least 0.1

mm. and at the most 0.5 mm., and less than I 50 per cent magnesia in the format fine meal a preponderant part of which is in a state of initial particle size not exceeding about 0.06 mm.

3. Refractory brick having great spalling resistance, slag resistance and strength at high fair strength at atmospheric of ground chromite freed from the finest fraction up to an upper limit of at the least 0.1 mm. and at the most cent magnesia in the form of fine meal, nearly is in a fine state of initial particle size not exceeding 0.02 mm.

0.5 mm., and less than 40 per sure.

7. In the manufacture of bricks from a mixture of raw chromite and magnesia the process which comprise grinding the chromite, eliminating from the ground material substantially the whole of the finest fraction which goes at least up to a grain size of 0.1 mm. and does not go beyond 0.5 mm., mixing from 60 to 80 parts of the residual batch with 40 to 20 parts of finely divided magnesia reduced by special fine grinding to such a degree of fineness that barticlesof initial size below 0.06 mm. are largely in excess of the others not exceeding asize of about 0.1 mm., and with a bonding substance, and molding the mixture into bricks under pressure.

8. In the manufacture of bricks from a mixture of raw chromite and magnesia the process which comprises grinding the chromite, eliminating from the ground material substantially the whole of the finest fraction which goes at least up to a grain size of 0.1 mm. and does not o beyond 0.5 mm., mixing from 60 to 80 parts of the residual batch with 40 to 20 parts of finely divided .magnesia in which particles below a size of 0.06 mm. are largely in excess, and with a bonding agent, molding the mixture intobricks under high pressure, and baking the bricks.

9. In the manufacture of bricks from a mixture of raw chromite and magnesia the process which comprises grinding the chromite, eliminating from the ground material substantially the whole of the finest fraction up to a limit of at least 0.1mm. and at the most 0.5 mm., mixing parts of the residual batch with 30 parts of finely ground magnesia, nearly half of which consists of meal of initial particle size not exceeding 0.02 agent, and molding the mixture into bricks under pressure.

, JOSEE BERLEIL' mm., and with abonding 

